From fbe0a3d3503698bda35c80f730c9c1162896738d Mon Sep 17 00:00:00 2001
From: Dominic Gannaway <trueadm@users.noreply.github.com>
Date: Wed, 5 Feb 2025 13:15:20 +0000
Subject: [PATCH] wip

---
 packages/svelte/src/index-client.js           |  2 +-
 .../internal/client/reactivity/resources.js   | 87 +++++++++++++------
 packages/svelte/types/index.d.ts              | 14 ++-
 3 files changed, 70 insertions(+), 33 deletions(-)

diff --git a/packages/svelte/src/index-client.js b/packages/svelte/src/index-client.js
index 589f4a59bb..ba1e2d3501 100644
--- a/packages/svelte/src/index-client.js
+++ b/packages/svelte/src/index-client.js
@@ -219,4 +219,4 @@ export { getContext, getAllContexts, hasContext, setContext } from './internal/c
 export { hydrate, mount, unmount } from './internal/client/render.js';
 export { tick, untrack } from './internal/client/runtime.js';
 export { createRawSnippet } from './internal/client/dom/blocks/snippet.js';
-export { Resource } from './internal/client/reactivity/resources.js';
+export { Resource, getResource, deferPending } from './internal/client/reactivity/resources.js';
diff --git a/packages/svelte/src/internal/client/reactivity/resources.js b/packages/svelte/src/internal/client/reactivity/resources.js
index f34092ee4d..c7ca25601a 100644
--- a/packages/svelte/src/internal/client/reactivity/resources.js
+++ b/packages/svelte/src/internal/client/reactivity/resources.js
@@ -1,12 +1,17 @@
 /** @import { Derived, Effect, Source } from '#client' */
 
 import { UNINITIALIZED } from '../../../constants.js';
+import { is_array } from '../../shared/utils.js';
 import { EFFECT_PRESERVED } from '../constants.js';
-import { active_effect, get, handle_error } from '../runtime.js';
+import { getContext, setContext } from '../context.js';
+import { active_effect, get, handle_error, untrack } from '../runtime.js';
 import { derived } from './deriveds.js';
 import { block } from './effects.js';
 import { internal_set, source } from './sources.js';
 
+const resource_symbol = Symbol('resource');
+const current_map = new WeakMap();
+
 /**
  * @template T
  */
@@ -18,14 +23,28 @@ export class Resource {
 	/** @type {Source<boolean>} */
 	#pending = source(true);
 
-	/** @param {() => Promise<T>} fn */
-	constructor(fn) {
+	/**
+	 * @param {() => Promise<T>} fn
+	 * @param {symbol} [symbol]
+	 */
+	constructor(fn, symbol) {
 		let parent = /** @type {Effect | null} */ (active_effect);
+		current_map.set(this, this.#current);
 
 		if (parent === null) {
 			throw new Error('TODO cannot create resources outside of an effect');
 		}
 
+		if (typeof symbol === 'symbol') {
+			let resources = getContext(resource_symbol);
+
+			if (resources === undefined) {
+				resources = new Map();
+				setContext(resource_symbol, resources);
+			}
+			resources.set(symbol, this);
+		}
+
 		/** @type {{}} */
 		var current_token;
 
@@ -59,39 +78,51 @@ export class Resource {
 		return get(this.#pending);
 	}
 
-	get current() {
-		var value = get(this.#current);
-
-		if (value === UNINITIALIZED) {
-			throw new Error('Resource is not yet resolved, ensure it is awaited');
-		}
-
-		return value;
-	}
-
-	get latest() {
-		var value = get(this.#current);
-		get(this.#pending);
-
-		if (value === UNINITIALIZED) {
-			throw new Error('Resource is not yet resolved, ensure it is awaited');
-		}
-
-		return value;
-	}
-
 	/**
-	 * @param {(arg0: { readonly current: T; readonly pending: boolean; readonly latest: T; }) => void} onfulfilled
+	 * @param {(arg0: { readonly current: T; readonly latest: T; }) => void} onfulfilled
 	 * @param {((reason: any) => PromiseLike<never>) | null | undefined} onrejected
 	 */
 	then(onfulfilled, onrejected) {
 		return get(this.#fn).then(() => {
 			var self = this;
 			onfulfilled({
-				get current() { return self.current },
-				get pending() { return self.pending },
-				get latest() { return self.latest }
+				get current() {
+					return get(self.#current);
+				},
+				get latest() {
+					get(self.#pending);
+					return get(self.#current);
+				}
 			});
 		}, onrejected);
 	}
 }
+
+/**
+ * @template T
+ * @param {symbol} symbol
+ * @returns {Resource<T> | null}
+ */
+export function getResource(symbol) {
+	return getContext(resource_symbol)?.get(symbol) ?? null;
+}
+
+/**
+ * @template T
+ * @template V
+ * @param {Resource<T> | Resource<T>[]} resources
+ * @param {() => V} fn
+ */
+export function deferPending(resources, fn) {
+	const res = is_array(resources) ? resources : [resources];
+
+	for (let i = 0; i < res.length; i += 1) {
+		const resource = res[i];
+		const pending = untrack(() => resource.pending);
+		get(/** @type {Source} */ (current_map.get(resource)));
+		if (pending) {
+			break;
+		}
+	}
+	return untrack(fn);
+}
diff --git a/packages/svelte/types/index.d.ts b/packages/svelte/types/index.d.ts
index 9d8bda9ad6..95485d8414 100644
--- a/packages/svelte/types/index.d.ts
+++ b/packages/svelte/types/index.d.ts
@@ -512,13 +512,19 @@ declare module 'svelte' {
 	 * ```
 	 * */
 	export function untrack<T>(fn: () => T): T;
-	export function isPending(fn: () => any): boolean;
+	export function getResource<T>(symbol: symbol): Resource<T> | null;
+
+	export function deferPending<T, V>(resources: Resource<T> | Resource<T>[], fn: () => V): V;
 
 	export class Resource<T> {
 		
-		constructor(fn: () => Promise<T>);
-		get current(): T | Promise<Awaited<T>>;
-		get latest(): T | Promise<Awaited<T>>;
+		constructor(fn: () => Promise<T>, symbol?: symbol | undefined);
+		get pending(): boolean;
+		
+		then(onfulfilled: (arg0: {
+			readonly current: T;
+			readonly latest: T;
+		}) => void, onrejected: ((reason: any) => PromiseLike<never>) | null | undefined): Promise<void>;
 		#private;
 	}
 	type Getters<T> = {